As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. The basis for these techniques is the notion that a sample may be examined by analyzing the reflected energy that results when an optical beam is directed at a sample. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.
One widely used type of optical metrology system includes a pump laser. The pump laser is switched on and off to create an intensity-modulated pump beam. The pump beam is projected against the surface of a sample causing localized heating of the sample. As the pump laser is modulated, the localized heating (and subsequent cooling) creates a train of thermal and plasma waves within the sample. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
To monitor the surface changes, a probe beam is directed at a portion of the sample that is illuminated by the pump laser. A photodetector records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. For most implementations, this is performed using a heterodyne or lock-in detector (See U.S. Pat. No. 5,978,074 and, in particular, FIG. 2 for a discussion of such a lock-in amplifier/detector). Devices of this type typically generate separate “in-phase” (I) and “quadrature” (Q) outputs. These outputs are then used to calculate amplitude and phase of the modulated signal using the following equations:Amplitude=√{square root over (I2+Q2)}  (1)Phase=arctan(Q/I)   (2)
The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for test samples can then be analyzed by comparison to the amplitude values obtained for the calibration samples.
Systems of this type (i.e., those using external means to induce thermal or plasma waves in the sample under study) are generally referred to as MOR (modulated optical reflectance) type systems. MOR-type systems are used to study a range of attributes, including material composition and layer thickness. MOR-type systems and their associated uses are described in more detail in U.S. Pat. Nos. 4,634,290, 4,636,088, 4,679,946; 4,854,710, 5,978,074 and 5,074,669. Each of these patents is incorporated in this document by reference. The assignee manufactures a commercial device, the Therma-Probe which operates using the MOR technique.
An important use of MOR-type systems is the measurement and analysis of the dopants added to semiconductor wafers before and after their activation. Dopants are ions that are implanted to semiconductors during a process known as ion implantation. The duration of the ion implantation process (i.e., total exposure of the semiconductor wafer) controls the resulting dopant concentration. The ion energy used during the implantation process controls the depth of implant. Both concentration and depth are critical factors that determine the overall effectiveness of the ion implantation process.
MOR-type systems are typically used to inspect wafers at the completion of the ion implantation process. The ion implantation damages the crystal lattice as incoming ions come to rest. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective surrogate for direct measurement of dopant concentration and depth.
In one approach, an MOR-type optical metrology tool with an advanced signal processing algorithm is used to record both quadrature (Q) and in-phase (I) components of the signal for a series of specially prepared calibration samples. The measurement method then performs a linear fit using the recorded points to define a calibration line within an I-Q plane. The slope of this line is defined by the implantation energy and the points along the line correspond to different dopant concentrations. Thus, the damage profile can be characterized by comparison of measured and calibration data in I-Q space. Characterization of samples using I and Q outputs is described in U.S. Pat. No. 6,989,899, assigned to the same assignee and incorporated here by reference.
Dopant activation after the ion implantation step is usually performed by rapidly heating and cooling the sample is a special chamber. This process is also known as annealing of semiconductor wafers. During the anneal, dopant ions diffuse away from the surface and form a concentration profile within the sample. The transition between the implanted region containing activated dopants and the non-implanted substrate is commonly referred to as a junction. For advanced semiconductor manufacturing, it is generally desirable for the implanted and activated region to be shallow, typically 500 Å or less. Devices of this type are generally referred to as having ultra-shallow junctions or USJ.
A number of techniques have been developed to characterize the effectiveness of USJ process. Destructive and contact methods include secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and spreading resistance depth profiling (SRP). These techniques are capable of providing detailed USJ profile information, but at the expense of a turnaround time that is usually measured in days or even weeks or at the expense of damaging the surface with contacts.
Alternately, U.S. Pat. No. 7,248,367, assigned to the same assignee and incorporated here by reference, describes a non-destructive MOR-type system for simultaneous measurements of junction depth (Xj) and abruptness. System of this type performs a series of measurements at different separations between the pump and probe beams followed by the analysis of measured data in I-Q space. Similar approach to measure USJ depth and abruptness is described in the following publications: L. Nicolaides et al., Rev. Sci. Instrum. 74(1), 586 (2003) and A. Salnik et al., Rev. Sci. Instrum. 75(6), 2144 (2004) incorporated here by reference.
The assignee of the subject invention has previously developed simultaneous multiple angles of incidence measurement tools which have been used to derive characteristics of thin films of semiconductor wafers and to obtain critical dimension measurements on grating structures. Detailed description of assignee's simultaneous multiple AOI devices can be found in the following U.S. Pat. Nos. 4,999,014; 5,042,951; 5,181,080; 5,142,473 and 5,596,411, all incorporated herein by reference. The assignee manufactures a commercial device, the Opti-Probe which takes advantage of some of these simultaneous multiple angles of incidence systems.
One of these simultaneous multiple angles of incidence tools is marketed by the assignee under the name Beam Profile Reflectometer (BPR). In this tool, a probe beam is focused with a strong lens so that the rays within the probe beam strike the sample at multiple angles of incidence. The reflected beam is directed to an array photodetector. The intensity of the reflected beam as a function of radial position within the beam is measured and includes not only the specularly reflected light but also the light that has been scattered into that detection angle from all the incident angles as well. Thus, the radial positions of the rays in the beam illuminating the detector correspond to different AOI on the sample plus the integrated scattering from all of the angles of incidence contained in the incident beam. In this manner, simultaneous multiple angles of incidence reflectometry can be performed.
Another tool used by the assignee is known as Beam Profile Ellipsometry (BPE). In one embodiment as shown and described in U.S. Pat. No. 5,042,951, the arrangement is similar to that described for BPR technology except that additional polarizers and/or analyzers are provided. In this arrangement, the change in polarization state of the various rays within the probe beam is monitored as a function of angle of incidence.
It is believed that the BPR/BPE technologies can be combined with the MOR measurement system to provide more information about the semiconductor sample under investigation.